Wall Bracing and the IRC

Wall bracing is one of the critical elements of a wood-framed
structure, providing resistance to forces that act along the wall
plane. In storm-prone coastal areas especially, braced walls help
the whole house resist lateral wind forces.

The higher those lateral wind loads are, the stronger the structure
must be. That's why in the zones closest to the coast, where design
wind speeds exceed 100 or 110 mph, building codes require new homes
to have an engineered design. The design process involves
calculating the anticipated wind loads based on the given
building's location, exposure, and dimensions, and then specifying
appropriate assemblies to resist those loads. For wood-framed
houses, the resulting design ordinarily includes shear walls built
using closely nailed structural panel sheathing and rugged
hold-downs.

But for typical buildings where expected winds loads are more
modest, builders can use prescriptive wall-bracing rules contained
within the International Residential Code. The IRC's Section
R602.10 calls for "braced wall lines" composed of "braced wall
panels." The location, width, and construction details of these
braced wall panels, as well as the materials that can be used to
build them, are spelled out in the code's text, tables, and
illustrations. Understanding the rules in Section R602.10 is
critical for builders seeking to streamline their profit margins by
optimizing the framing package, but it will also help those
building in higher wind zones understand what's involved in an
engineer's design.

Section R602.10 of the International Residential Code allows
wide flexibility in how builders can brace against wind loads, but
it's not a simple prescription. Here's a clear path through the
maze of options.

Defining Braced Walls

In practice, braced wall panels as understood by engineers and code
officials are nothing more than areas of framed wall containing no
door or window openings and which have let-in bracing, diagonal
board sheathing, or some sort of code-approved sheet material to
stiffen the structure against racking. In many cases, the only
difference between a shear wall and braced wall is the inclusion of
hold-down brackets on the shear wall.

Keep in mind that not all sheathed walls constitute a code-defined
"braced wall panels." To count toward the total bracing
requirement, a sheathed segment has to be tall enough to cover the
full wall height and wide enough to satisfy minimum strength
requirements. Sheathing above and below windows does not count.
Neither does sheathing on sections that are too skinny, such as
thin strips of wall between windows or at corners — tall,
narrow pieces of wall can't be depended on to provide the needed
bracing.

Eight options. The code explicitly allows
eight different wall-bracing methods, each involving a different
material (see "IRC Wall-Bracing Methods"). But that flexibility of
multiple bracing options also adds complexity to the prescriptive
measures allowed. Here, we're focusing primarily on what the code
calls "Method 3": bracing with wood structural panels, such as
plywood or OSB. The total wall area you must devote to bracing is
less if you use plywood and OSB than if you rely on other
materials. But this is not to imply that Method 3 is any better or
stronger than any of the other methods allowed; every method
described in the code will work. However, structural wood sheathing
offers builders wider flexibility in the building design,
particularly when it comes to the placement of window and door
openings (see "Comparing Wall-Bracing Options ").

IRC Wall-Bracing
Methods

Method 1: 1x4 let-in wood bracing or equivalent
metal bracing

Method 2: Diagonal board sheathing

Method 3: Wood structural panel sheathing

Method 4: 1/2-inch structural fiberboard sheathing

Method 5: 1/2-inch gypsum board

Method 6: Particleboard sheathing panels

Method 7: Portland cement plaster

Method 8: Hardboard panel siding

Any of the materials listed above can be used to make braced wall
panels as defined within the IRC. For Method 1 (let-in bracing),
braces must be applied at a 45° to 60° angle. For all other
methods except Method 5 (gypsum board) and the "fully sheathed"
version of Method 3 (wood structural panels), braced wall panels
must be a minimum of 4 feet long (measured horizontally along the
wall line). Gypsum board braced wall panels must be a minimum of 8
feet long (or 4 feet long, if gypsum board is applied to both
sides). But if buildings are fully sheathed with wood structural
panels, braced wall panels may be narrower than 4 feet in many
situations. For all braced wall panels, materials must be fastened
to framing as specified in the code.

Comparing Wall-Bracing Options

Engineered Shear Walls

Engineered shear walls are used when wind loads equal or exceed 100
mph (2006 IRC) or 110 mph (2003 IRC). An engineer must calculate
project-specific wind loads and specify the size, location, and
construction of any shear walls accordingly. Shear walls ordinarily
have tight nailing at edges and engineered hold-downs at each
segment. Shear walls can be spaced in multiple segments as narrow
as 271/2 inches for wind design.

IRC "Method 3"

The IRC's "Method 3" can be used for relatively low wind loads
below 100 or 110 mph. Plywood or OSB fastened to some areas of each
wall stabilizes the walls against racking from the force of the
wind. "Braced wall panels" (areas of clear wall covered with
plywood or OSB) must be at least 4 feet wide, extend to the full
wall height, and be located at or near each end of the wall (but no
more than 25 feet apart).

IRC "Fully Sheathed"

If a building is fully sheathed with plywood or OSB, the IRC allows
a reduction in the total area devoted to braced wall panels. Also,
sheathed areas narrower than 4 feet may be counted as braced wall
panels (minimum width depends on the height of the wall and the
height of adjacent openings). However, all corners must be
constructed as braced wall panels.

Limitations of prescriptive measures. Before you
start applying IRC Section R602.10's rules to your building, be
sure you don't need an engineer. Several factors may push a project
outside the IRC's scope:

• Wind-speed zone. To qualify within the 2000
or 2003 IRC, the house must be in a design wind-speed zone below
110 mph. In the recently published 2006 IRC, that threshold has
been lowered to 100 mph.

• Story height. Story height cannot be more
than 11 feet 4 inches (10 feet of stud height plus 16 inches for
floor framing). There is an exception in Section 3 of the IRC that
allows studs as high as 12 feet and still falls within the IRC's
scope. In that case, the builder has to increase the braced-wall
amount by 20%.

• Number of stories. The IRC is limited to
buildings with three stories or fewer.

If a project falls outside the IRC's scope, whether because the
building is too tall, the walls are too high, or the wind exposures
are too extreme, then an engineered design is required.

Complying with "Method 3"

Although the details get complicated, following IRC Section
R602.10's rules is simple in concept: You just have to provide
full-height braced wall panels of the required width, at the
required locations, accounting for at least the required minimum
percentage of the total wall length.

The dimensions and proportions of bracing panels may vary depending
on factors such as the wall's height, the size of any openings in
the wall, and the number of stories above the wall. But in all
cases, the braced wall panels need to line up within a "braced wall
line" (Figure 1). Offsets are allowed within limits: Individual
offsets may be as much as 4 feet, and the total offset from outside
to outside may be as much as 8 feet.

Figure 1. Braced wall lines may include
offsets. However, the maximum offset is 4 feet, and the total
offset from one side to the other may not exceed 8 feet.

All exterior walls have to be braced, and the distance between
braced wall lines cannot exceed 25 feet. Again, there is an
exception: Braced wall lines as far as 50 feet apart are allowed if
the total amount of braced wall panel area is increased to
compensate for the greater distance between the braced wall lines.
While not immediately apparent, this exception makes sense: If the
house is wider, it catches more wind and experiences a greater
total load. Think of a sail: The bigger the sail, the greater the
force. In a wide house, that translates to more work that the
braced walls have to do. For buildings up to 50 feet wide, you have
a choice: (1) Include an interior wall equipped with bracing, or
(2) increase the area of braced panels in the exterior walls. One
way or the other, the increased load has to be handled. If you
choose to beef up the exterior walls, the code provides a
proportional formula for increasing the total width of braced wall
panels to beef them up. When houses get wider than 50 feet,
however, you have just one choice: Include an interior braced wall
line, no matter what.

"Fully Sheathed" Structures

Besides the basic Method 3, there's another choice using OSB or
plywood: Fully sheathe the structure as described in paragraph
R602.10.5 of the IRC. For the buildings to qualify, all exterior
walls must be sheathed in plywood or OSB, including areas above and
below windows or doors, and there have to be sheathed panels (not
windows) at all corners.

When you fully sheathe the building, you can reduce the total
braced-wall amount by applying a multiplier of either 0.8 or 0.9,
depending on the height of openings in the wall. If the wall has
door openings (measuring up to 85% of the wall height), the total
bracing amount can be reduced by only 10%. If walls contain just
window openings (up to 67% of wall height), the total bracing
amount can be reduced by 20%.

But just as important, the fully sheathed method lets you count
full-height wall segments narrower than 4 feet as braced wall
panels, in many cases (see "Length Requirements for Braced Wall
Panels"). For example, for an 8-foot-high wall with typical-sized
windows, even a 24-inch-wide section counts toward the wall-bracing
total. This gives the builder greater leeway in placing window and
door openings in the wall, while still achieving the code-required
bracing.

Note: Braced wall panel "length" requirements for houses that
are fully sheathed depend on the height of the wall and the height
of adjacent openings (as a percentage of the wall's height).
"Length" here refers to the horizontal wall distance; on site this
may be commonly referred to as the braced panel's
"width."

There are trade-offs, however. For one thing, the fully sheathed
method complicates the issue of determining the minimum braced
panel width. The basic Method 3 sets a single, simple minimum width
for braced wall panels: They all have to be at least 4 feet wide.
By contrast, the rules for a fully sheathed building vary depending
on the wall height and on the size of adjacent openings.

And there's another caveat. Under the basic Method 3, braced wall
panels can be located as far as 12 feet from the end of the braced
wall line — bracing doesn't necessarily have to be located at
corners. But to use the fully sheathed method, the builder must
construct all exterior corners as braced wall panels of the
required minimum width, including minimum corner panel widths, as
shown in the table on page 54. This means that if a builder wants
to place windows or doors very close to building corners, he will
have to look back at the regular Method 3, and his braced wall
panels will have to make up a greater proportion of the total
wall.

APA's "Narrow Wall-Bracing
Method"

In modern house designs, even the relatively flexible rules for the
fully sheathed house may constrain the placement of windows and
doors too much for some builders. One common issue has been the
need to flank garage-door openings with braced-wall sections
measuring 4 feet or wider. The bracing is important, but the space
enclosed by making the whole garage wider by 4 feet or more is not
particularly useful, and the wider room spans boost the cost of
foundation and framing and may affect the way the house is sited in
relation to setbacks.

To address this problem, APA - The Engineered Wood Association, a
group representing plywood and OSB manufacturers, has developed and
tested alternatives that have now gained code acceptance. Only
allowed at garage doors in a fully sheathed house, the "APA Narrow
Wall-Bracing Method" (Figure 2) uses wood studs and OSB or plywood
sheathing, but with some different framing details. Key
elements:

• an overlapping header (headers must run
past door openings all the way to the far edge of the braced-wall
section);

The benefit to builders is that if they follow the new specs, they
can use braced-wall sections as narrow as 16 inches.

Figure 2. To address the particular problem of
braced walls next to garage doors, the newest IRC version now
includes this method developed by APA - The Engineered Wood
Association. Close nail spacing, an overlapping header that extends
to the end of the wall, and anchor bolts help offset the reduced
segment width.

APA's method is based on equivalency testing: APA tested some
already-permitted braced-wall sections to evaluate their strength,
and then designed new, narrower wall assemblies that could supply
the same tested structural capacity. APA's method was made part of
the 2004 Supplement to the IRC, and is also written into the new
2006 IRC. ~

The information in this article is based on staff research and
interviews by Ted Cushman with Vladimir Kochkin, research engineer
with the NAHB Research Center.